Make the underline placement of tab-style toggle buttons configurable

[Julien-Fischer]

Feature

There is currently no client property allowing to configure the underline placement of a tab-style JToggleButton on a per-component basis.
For example, tabs placement in JTabbedPane can be set to top, left, bottom, or right.
Tab-style JToggleButton could therefore support the same underline placements.

In addition with being consistent with the JTabbedPane API, adding this feature to the tab-style toggle buttons would allow users to create task bars with any orientation (horizontal, vertical) and position (top, left, bottom, right) on screen.

Code example

// paint the button in tab style
toggleButton.putClientProperty( FlatClientProperties.BUTTON_TYPE, "tab" );
// paint the underline on the right side of the button (provided by this PR)
toggleButton.putClientProperty( FlatClientProperties.TAB_BUTTON_UNDERLINE_PLACEMENT, SwingConstants.RIGHT );

Compatibility

If not specified, the underline placement of tab-style toggle buttons is bottom so that this PR is compatible with previous releases. The bottom placement is also probably the most popular option for this component.